The photocatalytic degradation of a series of six acid dyes (Direct Red 80, Direct Red 81, Direct Red 23, Direct Violet 51, Direct Yellow 27, and Direct Yellow 50) has been tested compared in terms of color removal, mineralization, and toxicity (Lactuca sativa L. test) after photocatalysis on immobilized titanium dioxide. The dyes were examined at their natural pH and after hydrolysis at pH 12. Results show that hydrolysis decreases strongly the efficiency of color removal, that full mineralization takes much longer reaction time than color removal, and that toxicity is only very partially reduced. Some structural parameters, related to the structure and the topology of the dye molecules, could be correlated with the apparent color removal rates at natural pH.